This class is composed of a group of exclusively marine organisms and
includes the corals and sea anemones. The coral animal, or _polyp_,
secretes a cup-shaped calcareous (limy) exoskeleton. This skeleton,
called a _corallite_, is usually divided by radial partitions called
_septa_. The polyp lives in the _calyx_, which is the central
bowl-shaped depression in the top of the corallite (fig. 11a).
_Solitary_ corals form an individual corallite for each polyp, and
because of their shape these may be given such names as “horn corals”
(_Lophophyllidium_, Pl. 15) or “button corals” (_Micrabacia_, Pl. 16).
_Colonial_ or _compound_ corals (Pl. 15) live together in colonies,
which are formed of many individual skeletons attached to each other
(fig. 11b), and the compound mass of coral skeletons formed in this
manner is called a _corallum_. Fossil corals commonly occur in many
marine limestones and in places constitute a large portion of the rock.
[Illustration: Fig. 11. Morphology and principal parts of corals. (a)
Solitary or “horn” coral. (b) Colonial or compound coral.]
a
Columella
Septum
Corallite
b
Calyx
Septum
Corallum
The class Anthozoa has been divided into several subclasses, but only
one, the Zoantharia, is of paleontological importance.